Micro-electromechanical system (MEMS) membranes are used in a number of different optical applications. For example, they can be coated to be reflective to highly reflective and then paired with a stationary mirror to form a tunable Fabry-Perot (FP) cavity/filter. It can also be used to define the end of a laser cavity. By deflecting the membrane, the spectral location of the cavity modes can be controlled.
The MEMS membrane is typically produced by etching features into a layer of material to form the pattern of the membrane. An underlying sacrificial layer is subsequently etched away to produce a suspended structure in a release process. Often the structural layer is silicon and the sacrificial layer is silicon dioxide. The silicon dioxide can be preferentially etched in hydrofluoric acid. The membranes can be constructed from various other material systems. In some cases, alternating layers of high and low index material are used to create a membrane.
Typically, membrane deflection is achieved by applying a voltage between the membrane and a fixed electrode. Electrostatic attraction deflects the membrane in the direction of the fixed electrode as a function of the applied voltage. This effect changes the reflector separation in the FP filter or cavity length in the case of a laser. Movement can also be provided by thermal or other actuation mechanism.
The high reflectivity coatings (R&gt;98%) and/or coatings in which the reflectivity varies as a function of wavelength (e.g., dichroism) require dielectric optical coatings. The optics industry has developed techniques to produce these high performance coatings and has identified a family of materials with well-characterized optical and mechanical properties. These coatings typically include alternating layers of high and low index materials. Candidate materials include silicon dioxide, titanium dioxide and tantalum pentoxide. These coatings are usually quite thick, greater than 3 micrometers (um).